24.8 Valuator step types

Accessor macro: Marpa_Step_Type MARPA_STEP_RULE

MARPA_STEP_RULE is the step type for for a rule node. The application should perform its equivalent of rule execution.

• The “child values” for this step will be in the stack locations from marpa_v_arg_0(v) to marpa_v_arg_n(v).
• The rule for this step will be marpa_v_rule(v).
• The result of this step should be written to stack location marpa_v_result(v). Typically, the result of this step is determined by executing the semantics for its rule on its child values.
• The stack location of marpa_v_result(v) is guaranteed to be equal to marpa_v_arg_0(v).

Accessor macro: Marpa_Step_Type MARPA_STEP_TOKEN

MARPA_STEP_TOKEN is the step type for a token node. The application’s equivalent of the evaluation of the semantics of a non-null token should be performed.

• The symbol ID of the token will be at stack location marpa_v_token(v).
• Libmarpa will already have a“token value” for the token in this step, as was set by the marpa_r_alternative() method. See marpa_r_alternative(). Libmarpa’s “token value” will be in stack location marpa_v_token_value(v).
• The result of the applications’s evaluation of the semantics of this token should be placed in stack location marpa_v_result(v). Often, an application will simply copy Libmarpa’s “token value” to stack location marpa_v_result(v).

Accessor macro: Marpa_Step_Type MARPA_STEP_NULLING_SYMBOL

MARPA_STEP_NULLING_SYMBOL is the step type for a nulled node. The application’s equivalent of the evaluation of the semantics of a nulled token should be performed.

• The ID of the nulled symbol will be at stack location marpa_v_symbol(v).
• The application’s value for this nulled symbol instance should be placed in stack location marpa_v_result(v). Often, an application will assign a fixed value to each nullable symbol, and will simply copy this fixed value to stack location marpa_v_result(v).

The use of the word “nulling” in the step type name MARPA_STEP_NULLING_SYMBOL is problematic. While the node must be zero-length (nulled), the node’s symbol need not be nulling: it may be a proper nullable. The name of the step type would more correctly be “MARPA_STEP_NULLED_SYMBOL”, but it is left as is for backward compatibility. See Nulling versus nulled.

Accessor macro: Marpa_Step_Type MARPA_STEP_INACTIVE

When this is the step type, the valuator has gone through all of its steps and is now inactive. The value of the parse tree will be in stack location 0. Because of optimizations, it is possible for valuator to immediately became inactive — MARPA_STEP_INACTIVE could be both the first and last step. Once a valuator becomes inactive, it stays inactive.

Accessor macro: Marpa_Step_Type MARPA_STEP_INITIAL

The valuator is new and has yet to go through any steps.

Accessor macro: Marpa_Step_Type MARPA_STEP_INTERNAL1

Accessor macro: Marpa_Step_Type MARPA_STEP_INTERNAL2

Accessor macro: Marpa_Step_Type MARPA_STEP_TRACE

These step types are reserved for internal purposes.